Question 22: A Ma’moom (person being led by an Imaam in Prayer) joined ‘Asr (Afternoon) Prayer during the fourth Rak‘ah (unit of Prayer) then another man came and started praying behind him. Is it thus permissible for the first praying person to change his intention from being a Ma’moom to being an Imaam (the one who leads congregational Prayer)?
Answer: It is permissible for whoever misses congregational Salaah (Prayer) and cannot find anybody else to start a new congregation with, to pray (as a Ma’moom) behind a person who is making up for the Rak‘ahs (units of Prayer) that they missed after the Tasleem (salutation of peace ending the Prayer) of the Imaam. Proof for the foregoing is that the Prophet (peace be upon him) was offering Salaah during the night and Ibn ‘Abbaas came and prayed behind him. Moreover, offering Salaah in congregation is more rewarded... read more here.